Eldfellite and Heklaite

Two new world minerals: Eldfellite and Heklaite

An international team of minerologists and geologists has been studying volcanogenic encrustations associated with three eruptions in Iceland – Surtsey 1963-1967, Eldfell 1973 and Hekla 1991. Among the minerals found at the eruption sites were 27 mineral species that had never before been found as natural minerals. Two of these have now been accepted as new world minerals: eldfellite and heklaite.

Volcanogenic encrustations form during or after volcanic eruptions: a motley skin or crust around crater rims, on the surface of lavas or in lava caves. Most encrustations are a direct product of magmatic degassing, formed as gases stream out of openings in cooling lava. Other encrustations are formed from steam discharge; encrustations of this type are found mainly in lava caves.

Encrustation field on the eastern eruptive fissure of the 1991 Hekla eruption, where heklaite was found. This site is now buried under scoria from the 2000 Hekla eruption. Photograph: Sigurður S. Jónsson.

X-ray powder diffraction analyses were performed on the encrustation samples at the University of Copenhagen Department of Geography and Geology and Iceland GeoSurvey (ÍSOR). The encrustations proved to be composed of numerous mineral species, some of them microscopic. At Surtsey, a total of 34 species were identified, the most common being gypsum, opal-A, calcite, halite, ralstonite and thenardite. Thirty one mineral species were identified at Eldfell and the Eldfell lava field, of which anhydrite, opal-CT, ralstonite, gypsum and hematite were the most common. Finally, 36 mineral species were identified in the encrustations from the 1991 Hekla eruption, the most common of which were ralstonite, opal-A, malladrite, hematite and two unknown minerals.

A total of 27 minerals found at these three sites had never before been found as natural minerals, although several had been previously created in laboratories. These unknown minerals have been divided into four groups: newly accepted world minerals (2 species); new, partially defined minerals (4 species); probable new minerals (5 species); and suspected new minerals (16 species).

The two mineral species that have been accepted as new world minerals come from Eldfell and Hekla.

Eldfellite(NaFe(SO4)2) was found on the northeast rim of Eldfell. It is monoclinic, with yellowish-green, platy crystals that average only 15x3 μm in size.

Heklaite(KNaSiF6) was found at the eastern eruptive fissure active during the 1991 Hekla eruption. It is orthorhombic. Crystals are colourless and average 40x42 μm in size.

Eldfellite, sample NI 13556, from the Eldfell volcano in the Westman Islands. The encrustations are a mixture of eldfellite (greenish yellow), tamarugite and an unknown mineral. A thin film of hematite gives these samples a reddish colour in places.

A total of 51 mineral species were identified on the encrustations, 32 of which had never before been found in Iceland. The IINH mineral database formerly contained 230 Icelandic mineral species (not counting varieties). Now, the total number of species in the database is 262 – a considerable increase in the number of mineral species known to be found in Iceland.

When researchers believe that they have found a new mineral, they must describe the mineral in great detail, following set procedures. The Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) then evaluates whether the mineral species in question represents a new world mineral.

These two mineral species are the first in the world to be named after an Icelandic locality. In 1917, a Danish mineralogist described an unusual sample from Teigarhorn in Berufjörður Fjord. It was at first taken to be a new mineral species, baptised flokite in honour of Hrafna-Flóki, who made an expedition to Iceland in the 9th century. More reliable tests made several years later showed that the mineral was, in fact, mordenite.